drm/exynos: rename s/HDMI_OVERLAY_NUMBER/MIXER_WIN_NR

HDMI_OVERLAY_NUMBER is specific of mixer driver and be used "windows
layer" term in exynos user manaual, so rename it.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>